In the first instance, we would be looking for plant life, rather than 'little green men'.The life forms most likely to develop on a planet would be bacteria. It is bacteria thathave generated the oxygen we breathe on earth.

　　19.In the first instance, we would be looking for plant life, rather than 'little greenmen'.

　　首先我们要寻找植物，而不是那种“小绿人”。

　　语言点1:句子结构分析:rather than的意思是“而不是”，在这句话中相当于instead of。

　　语言点2:这句话中的“小绿人”指代传说中的外星人。

　　20.The life forms most likely to develop on a planet would be bacteria.

　　行星上最容易生存下来的是细菌。

　　21.It is bacteria that have generated the oxygen we breathe on earth.

　　正是细菌生产出我们在地球上呼吸的氧气。

　　语言点:句子结构分析:这句话是个强调句。We breathe on Earth为定语从句。